1. Field of the Invention
This invention relates to controlled denitrification of fluids by anaerobic bacteria. More particularly, the invention relates to use of the oxidation-reduction (redox) potential ("ORP" or "eH") of the fluid medium to control the denitrification process. This invention has potential application to aquarium, aquaculture, and waste water treatment industries, and to any other closed or semi-closed system (e.g., space travel, closed biospheres, swimming pools, industrial effluents, etc.).
2. Description of Related Art
A bibliography with consecutively numbered references is included at the end of the specification. The superscripted numbers included throughout the specification refer to these references. The references listed in the bibliography, to the extent they provide exemplary procedural or other details supplementary to those set forth herein, are specifically incorporated herein by reference.
One of the end products of amino acid degradation is ammonia (NH.sub.3)..sup.8 As a result, protein catabolism results in a net production of ammonia, which aquatic organisms release directly into their environment..sup.22 In the open ocean, ammonia is normally taken up by photosynthetic organisms,.sup.6,7 but in most aquaculture facilities, it is not practical to rely on these micro- and macroalgae to remove ammonia..sup.16,18,24 Without some mechanism to remove it, ammonia can easily build up to toxic levels.
In most indoor, closed aquaculture systems, ammonia is oxidized to nitrite (NO.sub.2.sup.-) in an aerobic biofilter by autotrophic bacteria..sup.12,23 Nitrite is more toxic than the ammonium ion (NH.sub.4.sup.+), so a second set of bacteria are normally used to oxidize the nitrite to nitrate (NO.sub.3.sup.-). While nitrate is considerably less toxic than ammonium or nitrite, it too can become a problem at high levels.
Nitrate is typically removed from recirculating culture systems by water exchange. Unfortunately, water exchange has several drawbacks. First, water removal in aquaculture systems normally involves a slow exchange, with thorough mixing of old and new water to avoid stressing cultured animals..sup.20 The thorough mixing may result in a net loss of effectiveness of the exchange, since the discharged waste is already diluted by the newly introduced water. Second, in systems where natural sea water is unavailable, deionized water and sea salts must be mixed, which may incur heavy costs. Finally, the high nitrate effluent must normally be discharged.
This last condition is potentially the most troublesome, since salt water can not typically be discharged into a sewage system, nor dumped into a river. Further, there is growing environmental concern about the discharge of nitrogenous wastes. Discharge permits may be complex and often require very stringent pollutant limits..sup.6 It may be more economically feasible and environmentally conscientious to remove the nitrate and reuse the sea water than to discharge nitrate-laden effluent.
The ability of a compound to be oxidized is normally represented as an electrical potential relative to a hydrogen electrode..sup.2 This is the ORP and is normally measured in millivolts (mV). In water, oxidizing compounds are reduced as terminal electron acceptors in the electron transport chain..sup.4,5 They are generally utilized by bacteria in descending order of electropositivity, i.e., O.sub.2 first, then NO.sub.3.sup.-, NO.sub.2.sup.-, NO, SO.sub.4.sup.-2, etc..sup.1,23
This situation may be exploited to remove nitrate from water. Facultative anaerobic bacteria in anaerobic conditions, with sufficient carbon, can be induced to reduce nitrate to nitrite: EQU NO.sub.3.sup.- .fwdarw.NO.sub.2.sup.- Step 1
With sufficient time and available carbon, nitrite can be further reduced to nitrogen gas via the chain: EQU NO.sub.2.sup.- .fwdarw.NO.fwdarw.N.sub.2 O.fwdarw.N.sub.2 Step 2.sup.10,11
However, a system using bacteria to remove nitrate from water can be difficult to control and may require close monitoring to prevent release of toxic intermediate oxides of nitrogen, or worse, reduced sulfur as hydrogen sulfide (H.sub.2 S)..sup.1,23
Waste water treatment also typically uses bacterial denitrification for nitrogen removal..sup.3,9 Waste treatment, however, does not generally require such fine control as in the aquaculture context, since production of hydrogen sulfide and bacterial particulate is not normally a critical concern.
Some industrial treatment researchers have reported denitrification rates as high as 362 mg N/L.multidot.h in solutions with nitrate levels of 1000 mg/L..sup.17,19 Reported removal rates of these magnitudes were achieved in industrial water treatment by injection of acetate. It would seem that acetate is the food of choice, but bacteria populations fed with acetate may soon be dominated by Pseudomonas aeruginosa, a human pathogen..sup.17 Alcohols such as ethanol or methanol consistently support lower denitrification rates but select for a more acceptable bacteria population..sup.13
Waste water treatment often utilizes dissolved and particulate organic matter already present in the water (grey water) as a food source..sup.3 Denitrification in the aquaculture context, however, generally requires addition of a food source (such as methanol) because of the extremely low dissolved organic carbon levels of the typical culture system..sup.25 To ensure complete removal of nitrate and nitrite, typical waste water residence times range from 1 day to as long as several weeks..sup.14,19 Residence times of this duration are generally unsuitable for use in aquaculture because they can result in production of toxic H.sub.2 S.
Denitrification is the conclusive step in the removal of nitrogen bearing compounds from water..sup.9 In aquaculture, this has often been viewed as an impractical step..sup.20 However, there has been some work toward developing denitrification as a viable means of nitrate removal..sup.1,23
Large scale denitrification has typically been a poorly controlled process..sup.9 Several factors may affect production of undesirable toxic by-products. Lack of available reduced carbon, limited reaction time, low bacterial biomass or high inlet dissolved oxygen levels can lead to production of nitrite by single step reduction of nitrate, without nitrite reduction. Excessive reaction time or excess available carbon may result in reduction of sulfate (SO.sub.4.sup.-2) to hydrogen sulfide. Also, the close monitoring and fine control required by a biological denitrifier may be an impractical demand on human operators, who may succumb to fatigue or boredom.
It is a feature of this invention to provide a denitrification process and system which address at least some of the shortcomings experienced by prior art systems.